Where our figures come from

Capacity is volts times amps, and the working limit applies a 0.8 factor. In North America that factor is the NEC continuous-load rule, which applies to loads running three hours or more. Holiday lighting plainly qualifies.

In 230 volt regions there is no equivalent mandated derate. BS 7671 and AS/NZS 3000 size the circuit to the design current rather than applying a blanket percentage. We still apply the same 0.8 as practical advice, because outlet circuits are shared, and we say so rather than dressing it up as a regulation.

For North America we use 210 watts for 22 AWG cord and 420 watts for 20 AWG, which are the values UL listing conventionally applies to light strings and which manufacturers print on the tag.

For the UK and Ireland we use the BS 1362 plug fuse rating: 690 watts on a 3 amp fuse, 1,150 on a 5 amp, 2,990 on a 13 amp, all at 230 volts. Light sets are normally supplied with a 3 amp fuse.

For Australia and New Zealand, plugs are unfused, so there is no equivalent figure. The values we offer (400, 800 and 1,200 watts) are general cord ratings, and the manufacturer's printed maximum should always be used in preference.

Each wattage is a single planning figure taken from the range that manufacturers and lighting retailers quote for that product class, at or near the midpoint, biased upward where the range is wide. The wattage chart prints both, so you can see the figure we apply next to the range it came from. Real products vary by bulb count, spacing and efficiency, and a set bought this year may differ from the same model bought three years ago.

Where a range is wide (inflatables especially, which are mostly fan) we bias toward the upper end, because under-estimating load is the error with consequences.

We do not calculate anything upstream of the outlet. Breaker sizing, circuit design, conductor selection and anything inside a consumer unit or panel are outside what a web page should be telling you, and in most jurisdictions that work is notifiable.

We also do not give a single global answer to anything. Every calculation asks for your region first, because voltage and wiring practice change the result.

And where sources disagree badly enough that we cannot pick between them, we publish no figure rather than a plausible-looking wrong one. A number that is out by a factor of two is worse than no number, because it is the figure someone uses to decide whether a run is safe to switch on.